The head positioning system in many disk storage devices relies on reading embedded servo fields on the storage media. Typically the servo fields are positioned in radial “spokes” at equally spaced intervals at boundaries of sectors on the storage media. The spokes typically have a shape that corresponds with an arced path of a read/write head as it moves across the storage media surface. The embedded servo fields typically include an address mark which identifies a particular sector on the storage media and read/write head position data which identifies a particular track on the disc. The positioning system depends on accurate reading of the position data within the embedded servo field.
Under ideal, jitter-free and noise-free operating conditions, the read/write head senses an address mark after a fixed, nominal delay time since the last address mark. Under many operating conditions, however, the speed of the rotating media has significant jitter, and consequently the delay time between address marks has jitter. An address mark detector must be enabled over a time range wide enough to account for jitter in order to read address marks under these operating conditions. Noise is also present in reading the address mark, and there can be a failure to recognize the address mark because of the noise. With the combined effects of jitter and noise, there is a possibility of failing to detect an address marks, and also a possibility of erroneously reading an address mark. Either of these failures results in time delays in repositioning the read/write head for re-reading, and degraded performance of the disc drive for the user.
A method and apparatus are needed to perform address mark detection in a manner that indicates failures to detect address marks and that reduces erroneously read address marks. Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.